In a transmitter, most of the power is dissipated in the power amplifier (PA). High throughput signals have higher peak-to-average-power-ratios (PAPRs). To prevent distortion, it is generally desirable that the PA works at a higher power back-off, however this results in lower efficiency of the PA. There are two main solutions to improve the efficiency of the PA: (i) working in lower power back-off and using PA linearizer to compensate the distortion; and (ii) convert a varying-envelope input signal to a constant-envelope signal and use the PA in a high power efficiency region.
Delta sigma modulator (DSM) is one of the techniques to generate a constant-envelope signal from a varying-envelope signal. Generally, a DSM based transmitter consists of a two-level DSM unit that quantizes a time-varying input signal into a one-bit signal while shaping the quantization noise, a signal up-converter that shifts the frequency of the quantized signal to the desired radio frequency (RF) carrier frequency, and one PA (SMPA) that boosts the power of the RF signal before transmission. The PA is often a switch-mode PA (SMPA). Given that the quantized signal has two levels, the envelope of the RF signal feeding the SMPA is constant. As such, no output power back-off is required and the SMPA is driven at saturation at all times, which optimizes its efficiency without substantially affecting the linearity.
Three figures of merit are used to estimate the performance of the DSM based transmitters: coding efficiency (CE), signal-to-noise and distortion ratio (SNDR) and adjacent channel power ratio (ACPR). The CE is the ratio between the in-band signal power to the overall quantized signal power. For many state of the art DSMs, the CE is critically low. Thus, when the SMPA saturates there is a very low desired output power. The overall efficiency of DSM based transmitters can be estimated by the CE of the DSM times the peak efficiency of the SMPA, and as a result of generated quantization noise, the efficiency of two-level DSM based transmitters is very limited. Quantization noise also significantly degrades the ACPR and SNDR of the transmitter, preventing the signal from passing the mask. Known solutions that attempt to improve the SNDR and ACPR often result in a limited signal bandwidth.
There is a need for improved DSM based power amplifiers and transmitters.